Conventional solar battery charging systems employ a “back flow” or reverse current diode to prevent battery current from flowing back through a solar array in the absence of solar energy. Typically, a Schottky diode is used for this reverse current protection due to a low forward voltage drop inherent for Schottky diode operation. The forward voltage drop has a direct impact on charge efficiency such that the less power that is dissipated across the diode, the more charge power is delivered to the battery. The charge efficiency has been acceptable when dealing with macro solar charging systems since the battery charge voltages have been typically “high” in ratio to the Schottky diode forward voltage drop. However, this is not desirable in micro solar charging systems where the charge voltages are not high in ratio to the Schottky diode forward voltage drop. Therefore it would be desirable to provide a circuit that prevents battery back flow current having less forward voltage drop than a Schottky diode.
FIG. 1 illustrates a prior art circuit diagram disclosed in U.S. Pat. No. 6,713,989, entitled “Solarswitch” issued to the inventor of this application and incorporated herein in its entirety. Referring now to FIG. 1, an active backflow switch Q1 which serves to prevent current from the charged battery from flowing back into the solar array S1 34 when no solar illumination is present thus protecting the battery from discharge. The circuit is composed of a differential amplifier IC1 44 BB OPA349 and a N-channel enhancement mode MOSFET switch Q1 Siliconix Si2302DS for example. The battery B1 38 in this example is a 4.2 volt, 2500 mah lithium polymer and the solar array S1 34 is composed of two triple junction GaAs solar cells providing 250 ma of charge current at approximately 4.5 volts. Also a battery charge voltage regulator is normally present but is not shown for simplicity.
Since the differential amplifier IC1 is supplied operational current from the battery B1 38, positive and negative terminals, and is continually operating even when there is no solar luminance, differential amplifier IC1 is selected to have ultralow quiescent current ≈10 microamp such that there would be insignificant current draw from the battery B1 38. This has served to work appropriately for large capacity batteries 500 mah and above. However, battery types commonly used in various ultralow power energy harvesting applications may have capacity ratings of 1 mah or lower and will rapidly discharge while continuously operating a device such as IC1 with a quiescent current draw near 10 micro amps. Therefore, a backflow switch with zero quiescent current in the non-operational state (off state when no energy is being harvested) is required to prevent discharge of low capacity batteries of 500 mah or less in energy harvesting applications and the like.